Solar cells comprising monocrystalline, polycrystalline, or amorphous silicon semiconductors are currently used in electrical appliances such as handheld calculators and in household solar power generation. However, the manufacture of silicon solar cells employs processes requiring a high degree of accuracy such as plasma CVD and high-temperature crystal growth. These processes not only consume a large quantity of energy but also need a costly equipment involving generation of vacuum and, as a result, incur an increase in production cost.
Now, a dye-sensitized solar cell comprising an oxide semiconductor such as titanium oxide on which a sensitizing dye such as a ruthenium metal complex is adsorbed has been proposed as a solar cell that can be manufactured at low cost. A concrete example of such dye-sensitized solar cell comprises a negative electrode which consists of a top layer of titanium oxide semiconductor on which a ruthenium complex dye is adsorbed, a middle layer of transparent conductive indium-tin oxide, and a bottom layer of transparent insulating material such as a transparent glass or plastic plate, a positive electrode which consists of a layer of metal such as platinum or a layer of conductor formed on a transparent insulating material such as a transparent glass or plastic plate, and an electrolyte solution which is sealed between the two electrodes. When light falls on the dye-sensitized solar cell, absorption of light by the dye excites the electrons of the dye, the excited electrons move to the semiconductive layer and then to the transparent electrode at the negative electrode while the electrons coming from the conductive layer reduce the electrolyte at the positive electrode. The reduced electrolyte is oxidized by transferring the electrons to the dye and this cycle is considered to account for power generation by a dye-sensitized solar cell.
At the present time, a dye-sensitized solar cell shows a lower efficiency in converting incident light energy to electrical energy than a silicon solar cell and improvement of this conversion efficiency presents an important problem to solve in realizing practicable manufacture of a dye-sensitized solar cell. The efficiency of a dye-sensitized solar cell appears to be influenced by the characteristics of individual constituent elements and also by a combination of such elements and a variety of approaches have been tried to improve the efficiency. In particular, attention is given to the interaction between a sensitizing dye and titanium oxide functioning as a semiconductive layer and to the technique for preventing the electrons that have once been injected into titanium oxide from flowing back to the electrolyte solution.
The following documents are known prior to this invention.
Patent document 1: JP2001-223037 A
Patent document 2: JP7-500630 A1
Patent document 3: JP10-504521 A1
Patent document 4: JP2002-512729 A1
Patent document 5: JP2003-234133 A
Patent document 6: JP2000-228233 A
Patent document 7: JP2004-227920 A
Non-patent document 1: Chemistry Letters, 853, 1999
Non-patent document 2: Photochem. and Photobio. A, Chem. 164 (2004) 117
The non-patent document 1 describes a method for improving the fill factor of a dye-sensitized solar cell by dissolving the organic dye Eosin Y in methanol as an entrainer and letting a titanium oxide semiconductor electrode adsorb the dye in supercritical carbon dioxide at a pressure of 25 Mpa and a temperature of 493 K. The patent document 1 describes adsorption of a dye on a semiconductor in a supercritical fluid and the supercritical conditions used there are a pressure of approximately 8 Mpa and a temperature of 40° C. The patent document 5 also describes adsorption effected in a supercritical fluid. The non-patent document 2 and patent document 7 describe the possibility of sharply increasing the short-circuit current density of a dye-sensitized solar cell by adsorbing acetic acid on titanium oxide after adsorption of the dye.
The patent document 6 describes a method for adsorbing a dye and further describes a rinsing operation. However, it does not disclose a carboxylic acid treatment after the rinsing.